Field
This disclosure relates to small scale safety testing of materials and a method and instrument to objectively differentiate between reaction and non-reaction events. This is important to Government and commercial explosive manufacturers, explosive testing laboratories, as well as international testing groups responsible for determining if materials are safe for transport.
Description of the Related Art
Electrostatic discharge (ESD) is one of the most frequent and the least characterized causes of accidental explosions of energetic materials. To have reliable data on electrostatic spark sensitiveness of energetic materials is thus a critical component within the manufacturing process, in research and development, physical processing, handling, storage, and transportation.
ESD testing is used to determine the response of explosive material, commonly energetic, when subjected to various levels of electrostatic discharge energy. The test sample is placed on a holder and electrostatic energy, which is conventionally stored in a charged capacitor, is discharged through the test sample.
The electric spark sensitivity or electrostatic discharge sensitivity of an explosive may be defined as the amount of energy of an electric spark discharge that could cause either initiation, ignition or, depending on the level of electrostatic discharge energy, explosion of an energetic sample under test.
Presently, electrostatic discharge sensitivity tests and apparatuses in current use are designed and fabricated by the laboratories for their own use to evaluate in-process hazards. However, the basic components of an ESD tester include a high voltage supply, hardware for varying capacitance, an electrical charging circuit, a triggering circuit, an electrode assembly and an electrostatic voltmeter to measure the voltage. Approximately 10-30 mg of the sample is placed on a grounded conductive surface to ensure the discharge passes through the material and various methods are used to determine if a reaction has occurred. Differentiating between electrostatic discharge events above (i.e., “Go” events) and below (i.e., “No-Go” events) energy levels, the observation that a reaction has occurred in the material has conventionally required subjective determinations by users of the testing apparatus. This invention reduces or eliminates the subjective nature of these measurements and is important for developing a uniform standard among multiple laboratories determining the electrostatic-discharge sensitivity of materials.
Historically, users have observed tests and documented smoke, sample consumed, jetting (visible ejection of material), spark, flame trace (charred residue or burn marks), flash/flame, audible pop, load report/explosion, and/or hardware damage for every test completed. Given that the testing involves violent events in the routine operation, discernment of reactions is problematic. Additionally, human observation deteriorates based upon fatigue, distractions, and operating procedures. The human observation method of differentiating between “Go” and “No-Go” events has the advantage of requiring low upfront cost. However, this method is subjective and inconsistent because it relies on observation and memory; Therefore it can yield inconsistent results. Moreover, user observation in ESD testing contributes to physiological damage of the user's eyes due to repeated ultraviolet light exposure. Conventional test procedures require direct observation of numerous trials. Damage affects due to continued and repeated exposure to ultraviolet light could cause permanent damage, making users' observation an undesirable form of ESD testing.
Gas detection can also be used for differentiating between “Go” and “No-Go” events. Most explosive materials are nitrogen containing organic molecules that include carbon, hydrogen and oxygen. Decomposition products from these materials are carbon dioxide, carbon monoxide, nitrogen, and water. Gas analyzers have evolved to enable the quantification and identification of carbon dioxide and carbon monoxide as decomposition products. Many factors affect detection and resolution of gas analyzers such as mass spectrometers. Specifically, ambient air includes fluctuating amounts of each of the gases, humans exhale some of the products, and any fuel burning operation (gas/propane engines) will produce the same products. Although the gas-detection method provides quantifiable metrics, the resolution of the gas-detection method depends on environmental conditions, and the method requires direct access to the sample chamber during testing, which may obscure the sample from other detection methods. Moreover, the gas-detection of carbon dioxide and carbon monoxide method only works for organic explosives.
In addition to the gas-detection and user-observation methods, Sandia National Laboratories has developed a slow-camera method for differentiating between “Go” and “No-Go” events. There, a digital single-lens reflex camera (D-SLR) is programmed with a long duration shutter that captures all light emitted during a one second exposure of the D-SLR camera during which an ESD event occurs. The images reveal a sort of historical record of light that can be reviewed by an observer and determine whether or not a reaction was detected. The major obstacle to this method is that it relies on a subjective decision that arbitrarily rules out light emitted from system components (sparks from burning pieces of the ESD needle called “flyers”). Although the slow-camera method produces good quality images that can be archived and re-referenced, this method, like the user-observation method, relies on subjective decisions.
Safety Management Services Inc. (SMS) uses an alternative high-speed camera method for differentiating between “Go” and “No-Go” events. As instrumentation progresses, high speed video methods have been adapted to capture testing events. The added benefit of temporal information allows testers to see latent and duration information not included in the open shutter technique pioneered by Sandia National Laboratories. SMS has developed a semi-automated system to detect reactions, but still relies upon user input for reaction detection. The SMS system requires user definitions of positive reactions based upon features of spark, flame, and resultant buoyancy of particles and smoke and is thus still subjective.
Thus, conventional electrostatic safety testing of material involves observations of in-process hazards and subjective determinations of sensitivity. Tests are conventionally carried out with a human observer attempting to discern between a spark from electrostatic discharge (ESD) and decomposition of a material that may add light, sound, or smoke to the reaction. Advances have been made in the automation of these subjective responses, yet still require user input and baseline reference assessments. User fatigue, memory interference, and perspective affect the data of subjective testing and have relegated the data to only be valid when relatively compared to a well characterized standard.
There is a clear and distinct need to develop a method and apparatus that can consistently and reproducibly resolve the difference between non-reaction and reaction events without user input.